A cathode ray tube (CRT) has been widely used till now and has many advantages in views of performance and price. However, the CRT also has disadvantages in views of miniaturization and portability.
In order to replace the CRT, a light and slim liquid crystal display device (LCD) has been developed which can realize high brightness, large size, low power consumption, and low cost. The LCD has more excellent resolution than other flat display devices. Also, in displaying moving pictures, the LCD exhibits rapid response time comparable to the CRT.
The LCD includes a thin film transistor (TFT) substrate in which pixel electrodes and TFTs are formed, and a color filter substrate in which R, G and B color filter layers are formed. The TFT substrate and the color filter substrate are attached together, with a liquid crystal layer being interposed therebetween. The TFT substrate and the color filter substrate are fabricated through several mask processes.
FIG. 1 is a plan view illustrating a pixel region of a related art LCD.
Referring to FIG. 1, gate lines 11a and 11b for applying driving signals and data lines 13a and 13b for applying data signals are intersected together to define unit pixel regions. A TFT serving as a switching element is disposed at the intersecting region of the gate line 11a and the data line 13a. 
A pixel electrode 9 formed of transparent metal is disposed in the unit pixel region. The pixel electrode 9 electrically contacts the TFT and has one edge overlapped with the adjacent gate line 11b, thereby forming an auxiliary storage capacitor.
Also, a gate pad 12 receiving the driving signal from the circuit board is formed at an edge of the gate line 11a. A gate contact pad 22 is formed on the gate pad 12 so as to improve an electrical contact characteristic.
Likewise, a data pad 23 receiving the data signal from the circuit board is formed at an edge of the data line 13a. A data contact pad 33 is formed on the data pad 23 so as to improve an electrical contact characteristic.
FIG. 2 is a sectional view taken along lines I-I′, II-II′, III-III′ and IV-IV′ of FIG. 1, showing the pixel region, the data line, the storage region, and the gate pad region, respectively.
A method of fabricating the related art LCD will be described below with reference to FIG. 2.
Referring to FIG. 2, a gate metal layer is formed on a transparent insulation substrate 10. The gate metal layer is exposed, developed and etched through a first mask process to form a gate electrode 1 in a region I-I′ where a TFT is to be formed. Simultaneously, gate lines 11a and 11b and a gate pad 12 are formed.
Then, a gate insulating layer 2, an amorphous silicon layer, a doped amorphous silicon layer are sequentially formed on the insulating substrate 10 and are etched through a second mask process to form an active layer 4 on the gate electrode 1. The active layer 4 includes a channel layer and an ohmic contact layer.
Next, a metal layer is formed on the insulation substrate 10 in which the active layer 4 is formed, and a source electrode 6a, a drain electrode 6b, a data line 13a, and a data pad (not shown) are formed on the active layer 4 through a third mask process.
After forming the source electrode 6a and the drain electrode 6b, a passivation layer 8 is formed on the insulation substrate 10, and a contact hole process is performed to open the drain electrode 6b, the gate pad and the data pad through a fourth mask process.
After the contact hole process, a transparent metal is formed on the insulation substrate 10, and a pixel electrode 9, a gate contact pad 22 and a data contact pad are formed through a fifth mask process, thereby completing the TFT substrate of the LCD.
However, when the TFT substrate is fabricated through the five-mask process, the fabrication process is complicated and the manufacturing cost increases. Specifically, the complicated fabrication process increases the defect rate and decreases the production yield.
Also, a 4-mask process has been developed to form the source electrode, the drain electrode and the active layer through a 1-mask process. In such a process, an active layer wider than a data line width is inevitably formed under the data line, resulting in the degradation of the image quality.
When the LCD is driven, a tail-shaped defect along the data line (which is called an active tail defect) is detected.
Also, comparing with the 5-mask process, the 4-mask process causes a problem of producing a bad quality pattern due to the height difference with the thereafter formed line as height of the source/drain electrode of the 4-mask progress is increased.